Sic is (1) a Latin-derived word used to indicate that quoted text is reproduced as it originally appeared, and (2) a verb meaning to set upon or incite to hostile action. So, for example, you might sic your dog on a snake. Sick, which is never a verb (except, rarely, in the colloquial phrasal verb sick up), would be incorrect in this sense.
Sic is inflected as sicced, siccing, and sics. Your spell-check might say these forms are wrong, but don’t let that stop you from using them.
Examples
Sick is sometimes misused in place of sic—for example:
… Harper’s office sicked the Mounties on her in connection with some outlandish and unproven allegations. [Straight.com]
This includes informational interviews, marketing professors sicking their students on me for a Charlie Rose style Q&A … [Ad Age]
But most high-standards publications use sic, not sick, to mean attack or incite to hostile action—for example:
Now in Morocco, the Jays sicced monkeys on the girls as they toured a Marrakech market. [New York Magazine]
Police say a man who tried to sic his pit bull on a neighbour ended up being attacked by his own animal. [Toronto Sun]
Yesterday he learned that the Quebec government is siccing its anti-corruption unit on city hall. [Montreal Gazette]
